In the case of a mobile radio system that comprises a base station with two antennas and a mobile radio subscriber, a fixed phase relationship is set up between the two transmission channels, which are to be assigned in each case to one of the antennas of the base station. In the UMTS (Universal Mobile Telecommunications System) standard, a so-called CLTD (Closed Loop Mode Transmit Diversity) function can be used to influence the relationship of the signals emitted by two antennas in order thereby to achieve constructive interference of the two transmission channels. The CLTD function can be operated in a mode 1 and a mode 2. The phase of one of the two antennas can be varied in mode 1. The phase of the other antenna remains fixed. The overall result is to vary the phase relationship between the two antennas. In addition to the variation, known from mode 1, in the phase relationship, mode 2 provides a variation in the amplitudes of the signals emitted from the antennas. The amplitudes of both antennas can be varied in mode 2.
The CLTD function is described in the 3GPP TS 25.214 UMTS specification. This specification is referenced repeatedly below. All data refer in this case to the V4.2.0 (2001-09) version.
FSM (Feedback Signalling Message) data words that are transmitted to the base station are formed in the mobile radio terminal from estimated channel impulse responses by means of the CLTD function. The channel impulse responses of both channels are always used in this case for an FSM data word. The FSM data words include information specific to the base station and relating to the optimum phase relationship and, if appropriate, relating to the optimum amplitudes of the signals to be emitted from the two antennas.
Two antenna weightings w1 and w2 that are applied to the signals which are to be emitted from the two antennas are formed in the base station from an FSM data word. The antenna weightings w1 and w2 are complex and have the following form:w1=α1+jβ1  (1)w2=α2+jβ2  (2)
Two diversity components are evaluated by multiplying the signals to be emitted by the antenna weightings w1 and w2. In this case, the antenna weightings w1 and w2 are selected with the aim of maximizing per slot the energy received by the mobile radio terminal while taking account of the weight quantization prescribed in the UMTS standard. This is equivalent to maximizing a proportionality factor P that is given by the following equation:
                    P        =                                                                              w                  →                                H                            ⁡                              [                                                                                                    H                        11                                                                                                            H                        12                                                                                                                                                H                        21                                                                                                            H                        22                                                                                            ]                                      ⁢                          w              →                                =                                                                      w                  →                                H                            ⁡                              (                                                      ∑                                          m                      =                      1                                        M                                    ⁢                                                            H                      m                      H                                        ⁢                                          H                      m                                                                      )                                      ⁢                          w              →                                                          (        3        )            
Here, the index m denotes the respective base station m (m=1, . . . , M). Furthermore, it holds for the matrix Hm and the vector w that:Hm=[{right arrow over (h)}m,1,{right arrow over (h)}m,2]  (4){right arrow over (w)}=[w1,w2]T  (5)
The indices 1 and 2 relate to two antennas i of a base station m. {right arrow over (h)}m,i are complex (Nm×1) vectors that stand for the channel impulse responses estimated by the channel estimator with a channel depth Nm. Each vector {right arrow over (h)}m,i is to be associated with one of the antennas i of the base station m. It is to be assumed without loss of generality for the following considerations that only one base station m is sending (M=1). The index m is therefore omitted below, thus simplifying equation (4):H=[{right arrow over (h)}1,{right arrow over (h)}2]  (6)With i, j=1, 2, it holds for the elements Hij of the matrix from equation (3) that:
                                          H                          i              ⁢                                                          ⁢              j                                =                                                                      h                  →                                i                H                            ⁢                                                h                  →                                j                                      =                                                                            H                                      i                    ⁢                                                                                  ⁢                    j                                                                              ⁢                              ⅇ                                  jφ                                      H                                          i                      ⁢                                                                                          ⁢                      j                                                                                                          ,                            (        7        )            
|Hij| specifying a modulus and φHij phase angle.
Consequently, the technical problem to be solved includes a determination of the antenna weightings w1 and w2 per slot in such a way that the proportionality factor P is maximized for given estimated channel impulse responses {right arrow over (h)}i=[hi,1, . . . , hi,n, . . . , hi,N]T. The boundary conditions for the value ranges of the antenna weightings w1 and w2 are to be observed in this process.
The following boundary conditions hold for the antenna weightings w1 and w2 in mode 1 of the CLTD function:
                              w          1                =                  1                      2                                              (        8        )                                          w          2                =                              1                          2                                ⁢                      ⅇ                          j              ⁢                                                          ⁢                              φ                2                                                                        (        9        )                                          φ          2                ∈                  {                      0            ,                          π              2                        ,            π            ,                          -                              π                2                                              }                                    (        10        )            
In this case, the phase angle φ2(s) is a function of the slot index s (s=1, 2, . . . , S). More detailed explanations of this are to be found in section 7.2 of the 3GPP TS 25.214 V4.2.0 UMTS specification.
The following boundary conditions hold for the antenna weightings w1 and w2 in mode 2 of the CLTD function:
                              w          1                =                              E            1                                              (        11        )                                          w          2                =                                            E              2                                ⁢                      ⅇ                          j              ⁢                                                          ⁢                              φ                2                                                                        (        12        )                                          E          1                ,                              E            2                    ∈                      {                          0.2              ,              0.8                        }                                              (        13        )                                                      E            1                    +                      E            2                          =        1                            (        14        )                                          φ          2                ∈                  {                      0            ,                          π              4                        ,                          π              2                        ,                                          3                ⁢                π                            4                        ,            π            ,                          -                              π                4                                      ,                          -                              π                2                                      ,                          -                                                3                  ⁢                  π                                4                                              }                                    (        15        )            
Here, the amplitudes E1(s) and E2(s) as well as the phase angle φ2(s) are functions of the slot index s (s=1, 2, . . . , S). Mode 2 is described in more detail in section 7.3 of the 3GPP TS 25.214 V4.2.0 UMTS specification.
To date, the optimum values for the phase angle φ2(s) and, if appropriate, for the amplitudes E1(s) and E2(s) have been determined as a function of the mode and slot index s by parameterizing equation (3) or by substituting the values in question. In this process, it is necessary in mode 1 to make a selection from only two phase angle values per slot. By contrast, it is necessary in mode 2 to evaluate 16 combinations of amplitude and phase angle. This problem has been solved to date by means of a digital signal processor because of the structure of the calculating algorithm. In this process, the digital signal processor generates on the output side an FSM data word which consists of FSM bits and includes information relating to the optimum values for the phase angle φ2(s) and, if appropriate, for the amplitudes E1(s) and E2(s).